Superconducting magnets are used in a variety of contexts, including nuclear magnetic resonance (NMR) analysis, and magnetic resonance imaging (MRI). To realize superconductivity, the superconducting magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the superconducting magnet includes one or more electrically conductive coils which are disposed in a cryostat containing a substantial volume of a cryogenic fluid such as liquid helium. Many such superconducting magnets operate in “persistent mode.” A superconducting magnet which operates in persistent mode is initially energized with current from an external power supply to start up its magnetic field, and then the power supply is disconnected from the superconducting magnet and the superconducting magnet maintains the current and the magnetic field due to its superconductivity.
Although a continuous supply of power is typically not required for the superconducting magnet to sustain the magnetic field, power (e.g., AC Mains power) is still supplied to a compressor which drives a cooling unit or “cold head”—herein referred to as a “cryocooler”—in order to maintain the temperature of the superconducting magnet near absolute zero so that the magnet's superconductivity persist.
Unfortunately, it is possible that the power to the cryocooler may be lost, for example, during an electrical power outage due to a storm. Furthermore, from time to time, the cryocooler may experience some malfunction or need to be turned off to perform maintenance.
When the cryocooler ceases to operate, conditions within the cryostat can degrade and the temperature of the superconducting magnet may begin to rise. At a certain point, if operation of the cryocooler is not reestablished to restore cooling of the superconducting magnet's environment, then the superconducting magnet's temperature will rise to reach a critical temperature where the superconducting magnet will “quench” and convert its magnetic energy to heat energy, thereby heating the cryogenic fluid within the cryostat. This will cause some or all of the cryogenic fluid to evaporate and be lost. Furthermore, the heat may damage the magnet and/or other components of the apparatus.
In that case, once operation of the cryocooler is reestablished, to return the magnet to superconducting operation may require replacing lost cryogenic fluid within the cryostat, cooling the magnet back down to below the critical temperature, connecting leads to the magnet to reapply current from an external power supply to the magnet so as to regenerate the magnetic field, and disconnecting the magnet for the external power supply again. Furthermore, if heat from the quench caused the magnet or other components to be damaged, they may need to be repaired or replaced.
This recovery process can be expensive and time-consuming. Typically, a trained technician must be dispatched to the facility (e.g., a medical center or hospital) where the superconducting magnet system is located and new cryogenic fluid (e.g. liquid helium), which may be quite costly, must be supplied to the cryostat. Further, supplies of helium are becoming increasingly limited and so the cost of lost helium can significant.
Although this can be a significant problem in a typical superconducting magnet system for an MRI apparatus, the problem can be at least somewhat ameliorated by the fact that such superconducting magnet systems typically employ a relatively large volume of cryogenic fluid (e.g., 1000 liters of liquid helium). The large volume of cryogenic fluid has a large thermal mass which may mean that power must be lost for a relatively long time—maybe even days—before the temperature of the magnet reaches the critical temperature and produces a quench. Furthermore, such systems typically have an access means by which a user may add cryogenic fluid to the cryostat from time to time to replace lost of evaporated cryogenic material.
However, some newer MRI apparatuses are being developed and deployed which employ so-called “cryofree” superconducting magnet systems which are closed or sealed and which may not include means for a user to add new cryogenic material to the system. Furthermore, such closed systems typically have a much smaller volume of cryogenic material in their cryostats than the conventional systems described above (e.g., only one liter, or a few liters, of liquid helium).
Accordingly, a quench in a cryofree or sealed superconducting magnet system may occur in only 30 minutes or less after the cryocooler suffers a loss of electrical power or some malfunction, or undergoes maintenance, which prevents it from operating properly and maintaining the temperature in the cryostat to be near absolute zero. Furthermore, since no means is typically provided for a user to add new cryogenic material to the system, if the cryogenic fluid is degraded or evaporated due to a quench, then recovery may require days or weeks.
One aspect of the present invention can provide an apparatus including: one or more gravity-fed cooling tubes configured to have therein a first cryogenic fluid for cooling a superconducting magnet; a first heat exchanger configured to transfer heat from the one or more gravity-fed cooling tubes to a second stage element of a cryocooler, wherein the first heat exchanger is configured to store therein a volume of a second cryogenic fluid; a storage device having an input connected to the first heat exchanger and configured to receive and store a boiled-off gas from the first heat exchanger when the cryocooler stops operating; and a second heat exchanger configured to transfer heat from the storage device to a first stage element of the cryocooler.
In some embodiments, the apparatus further includes an enclosure, and a thermal shield disposed within the enclosure, the thermal shield defining an inner region, and further defining a vacuum space between the thermal shield and a wall of the enclosure, wherein the one or more gravity-fed cooling tubes, the first heat exchanger, the storage device, and the second heat exchanger are disposed within the inner region.
In some versions of these embodiments, the apparatus further includes a thermal regenerator having an input connected to the output of the storage device and having an output connected to an outside of the enclosure
In some versions of these embodiments, the thermal regenerator is at least partially disposed in the vacuum space between the thermal shield and the wall of the enclosure.
In some versions of these embodiments, the apparatus further includes a second storage device disposed outside the enclosure and connected to the output of the thermal regenerator.
In some embodiments, the apparatus further includes a cold plate configured to transfer heat from the second heat exchanger to the first stage element of the cryocooler.
In some versions of these embodiments, the apparatus further includes a persistent current switch connected across the superconducting magnet; and at least one high temperature superconducting electrical lead having a first end connected to the superconducting magnet and having a second end connected to the cold plate.
In some embodiments, the storage device has a capacity for storing at least 3 liters of the boiled-off gas.
In some embodiments, the first stage element of the cryocooler is configured to operate at a first temperature and the second stage element of the cryocooler is configured to operate at a second temperature which is less than the first temperature, and the apparatus further includes a thermal switch which is configured to transfer heat from the first heat exchanger to the first stage element of the cryocooler when the first heat exchanger has a temperature which is greater than the first temperature, and which is configured to prevent a transfer of heat from the first stage element of the cryocooler to the first heat exchanger when the temperature of the first heat exchanger is less than the first temperature.
Another aspect of the invention can provide an apparatus including: one or more gravity-fed cooling tubes configured to have disposed therein a first cryogenic fluid for cooling a superconducting magnet; and a heat exchanger configured to have stored therein a volume of a second cryogenic fluid including a cryogenic liquid, wherein the heat exchanger is configured to transfer heat from the one or more gravity-fed cooling tubes to a cryocooler.
In some embodiments, the apparatus further includes a storage device having an input connected to the heat exchanger and configured to receive and store a boiled-off gas from the heat exchanger.
In some versions of these embodiments, the apparatus further includes a thermal regenerator having an input connected to the output of the storage device.
In some versions of these embodiments, the gravity-fed cooling tubes, the heat exchanger, and the storage device are disposed within an enclosure, and the thermal regenerator has an output that is connected to an exterior of the enclosure.
In some versions of these embodiments, the apparatus further includes a second storage device disposed outside the enclosure and connected to the output of the thermal regenerator.
In some versions of these embodiments, the cryocooler has at least a first stage element which is configured to operate at a first temperature and a second stage element which is configured to operate at a second temperature which is less than the first temperature, and the apparatus further includes a second heat exchanger configured to transfer heat from the storage device to the first stage element of the cryocooler, wherein the heat exchanger is configured to transfer heat from the one or more gravity-fed cooling tubes to the second stage element of the cryocooler.
In some embodiments, the cryocooler has at least a first stage element which is configured to operate at a first temperature, and the apparatus further includes a thermal switch configured to transfer heat from the heat exchanger to the first stage element of the cryocooler when the heat exchanger has a temperature which is greater than the first temperature, and which is configured to prevent a transfer of heat from the first stage element of the cryocooler to the heat exchanger when the temperature of the heat exchanger is less than the first temperature.
Yet another aspect of the invention can provide a method including: transferring heat from a superconducting magnet to a first cryogenic fluid disposed within one or more gravity-fed cooling tubes; and transferring heat from the first cryogenic fluid in the one or more gravity-fed cooling tubes to a cryocooler via a heat exchanger which has a second cryogenic fluid, including a cryogenic liquid, disposed therein.
In some embodiments, the method further includes providing a boiled-off gas from the heat exchanger to a storage device configured to store at least some of the boiled-off gas therein.
In some versions of these embodiments, the cryocooler has at least a first stage element which is configured to operate at a first temperature and a second stage element which is configured to operate at a second temperature which is less than the first temperature, and the heat exchanger transfers heat from the one or more gravity-fed cooling tubes to the second stage element of the cryocooler, and the method further includes transferring heat from the storage device to the first stage element of the cryocooler.
In some versions of these embodiments, the method further includes supplying at least some of the boiled-off gas from the storage device to a thermal regenerator.